Because Columbus had a rather unique combination of geographic ineptitude, and drive. He believed the globe was a lot smaller than it really is. Most educated people at the time knew better.
I alway heard it the other way, He knew the circumstance of the earth but mis-figured the distance from Paris to Peking [21,000

].
Columbus Married the Daughter of a large Portuguese Family of Navigators. As part of Her Dowry He got access to all the Family's Secret Logbooks.
So He knew about the Canaries and Cape Verde islands from his own voyages, and the logbooks gave him the Azores and Madrids. along with the sailing times.
There were also hints in the logbooks about land to the West, So He wasn't sailing Blind.
So take Chris out of the Picture -- The Daughter marries a young upcoming Portuguese Navigator from a rival Family, Helping unite them and keeping all the secret info in the Family.
Several years later a Portuguese Ship rounding the Gold Coast gets blown SW by a Storm.
The Navigator notes in his log -- that at the same latitude as the Kongo, Sighted a large Jungle Island 5 days sailing to the west. lots of Streams, Stopped and refilled the water Barrels, nothing else of Interest.
The ship continues to the East Indies.
Upon it return to Portugal the Navigator is hoisting a Few with some fellow Navigators to celebrate the safe return.
He mentions the western Island, and a couple others mention they have seen it also. The other Navigators take note of the discussion.
Two years later the ship with one of the others is blown west, The Navigator tells the Captain that there are good stream for the water barrels, but nothing else of Interest.
The Captain accepts that the Navigator knows about this, while the young Assistant Navigator notes this in his new Log.
Within several Years, the Western Island is a open Secrete among the Navigators, and is spread across Europe.
However Argon is busy with it's Italian Processions, and Castile is busy pushing the Requestia into NAfrica.
Sometime around the mid 1500's France send a expedition to check out the Island, as Portugal has the African coast locked up.